Erasing letters on Shabbos is only prohibited min HaTorah (from the Torah) when it's being done in order to write new letters. However if it's not being erased in order to write new letters then the prohibition is only m'drabbanan (Rabbinical.)
What about in a case where someone erases only "part" of a letter, however leaves the appearance (tzurah) of the letter completely in tact. Is this ossur m'drabbanan or is it perhaps even "muter" (permitted.) What's the proof either way?